President Donald Trump’s first speech at the United Nations today was fearsome.

It was also pretentious because Trump seems to think the he speaks for all Americans and that the US is the most respected voice at the UN. Neither of those thoughts is true since his rise to power.

His speech may deserve the term “historic” if world leaders, including long-time allies, now conclude that the US will definitely turn towards nationalism and away from cooperation and multilateral solutions for global problems.

Within America, the jury is still out on what Trump actually means by his unusual use of words and what actions he might contemplate on almost any issue.

His performance at the UN fueled similar perplexity. The speech suggested that the US may be an unreliable friend under Trump.

It seemed to confirm German Angela Merkel’s earlier conclusion that each country should look out for itself in a Trumpian world marked more clearly by nationalism.

But it could also have been only Trump-style declamation draped in dramatic words that his actions and decisions will not match. That mismatch has happened on several highly-charged issues like Obamacare, infrastructure spending, tax cuts, refugees, immigration and the wall isolating Mexico.

Whatever his true intentions, the dozens of world leaders at the193-nation UN General Assembly cannot prudently dismiss his threats to totally destroy a UN member with 25 million people.

Nor can they set aside as a retrograde vision, his assertions about nationalism and sovereignty. His views echoed the 19th century system of nation states competing for self-aggrandizement and dominance over others, which engendered the two World Wars of the 20th century.

His thoughts sounded like the self-centered nationalism of Russia’s Vladimir Putin, China’s Xi Jinping and India’s Narendra Modi. They, too, are determined to vastly increase the nationalistic sovereign military and economic power of their countries.

French President Emmanuel Macron, a close ally in theaters where the US military is engaged, reacted quickly. He talked down national sovereignty in favor of more cooperation and pooled sovereignty to solve problems across national frontiers, such as terrorism and climate change.

Like Germany, another vital US ally, his country belongs to the 28-nation European Union (27 after Britain leaves in 2019), whose survival depends on collaboration across borders. EU members are bound by treaty to give priority to many EU-wide regulations over national laws.

UN secretary-general António Guterres, a fervent advocate of stronger global cooperation, warned that “Trust within and among countries is being driven down by those who demonize and divide”.

Lamenting against “closed doors and open hostility”, he asked countries to treat refugees with “simple decency and human compassion.”

In contrast, Trump thundered in biblical tones that some parts of the world “are going to hell”.

“If the righteous many do not confront the wicked few, then evil will triumph… When decent people and nations become bystanders to history, the forces of destruction only gather power and strength,” he claimed.

His words on North Korea were radical. “The United States has great strength and patience, but if it is forced to defend itself or its allies, we will have no choice but to totally destroy North Korea. Rocket Man (North Korean leader Kim Jong Un) is on a suicide mission for himself and for his regime,” he threatened.

Adding, “No nation on Earth has an interest in seeing this band of criminals arm itself with nuclear weapons and missiles.”

He was uncompromising on Iran. “The Iran deal was one of the worst and most one-sided transactions the United States has ever entered into… Frankly, that deal is an embarrassment to the United States, and I don’t think you’ve heard the last of it, believe me.”

If he means what he says and acts accordingly, the world may be headed towards mayhem in the Far East and Middle East in the name of protecting the American people across the oceans.

That such words may be pretentious was signaled by Macron. He said the Iran deal was “solid, robust and verifiable”, and “essential for peace”.

About North Korea, he added, “France rejects escalation and will not close any door to dialogue.”

These views are likely to be echoed in coming days by almost all major leaders at the General Assembly.

In effect, that would leave Trump isolated and short of friends to carry forward the kind of nationalism towards which he may wish to lead the world.

Yet, all world leaders are forced to take his bellicose statements seriously. He is the freely elected leader of the nation with the world’s largest economy at $16 trillion and military capabilities to deliver awesome destruction anywhere.

They would be foolish to presume that he may not entirely mean what he seems to say. His many incredulous opponents during last year’s presidential campaign treated his words as going beyond his thoughts, and lived to regret it.

Both his opponents and his supporters, some of whom are disappointed by his backtracking on several issues, have the luxury of “wait and see”. They could even depose him at the next elections.

Things are very different when foreign leaders are involved. The UN General Assembly is not a US townhall packed with handpicked supporters who applaud on cue by campaign handlers.

There, serious leaders of their billions of citizens get together for a few days each year to find issues of common concern on which they can cooperate for solutions.

They may be democrats, dictators or tyrants but at the General Assembly, they try to assess how far they may cooperate with others without compromising their authority inside their own countries.

Trump’s speech has not reassured friends or discomfited enemies. Friends wonder whether his administration will push them in directions they do not want and enemies may be encouraged to prepare for divisions among long-standing friends of pre-Trump US foreign policy.